The present invention, a process for the chemical stabilization of heavy metal dusts and sludges, has particular utility for the steel industry, where control of furnace emissions are necessary. While this invention has broad application in the stabilization of heavy metals, it will be described in detail by its preferred use or application.
The basic or dominant steelmaking practice followed today in the domestic and foreign steel industry is the basic oxygen process. Such process utilizes molten pig iron as the basic charge to the furnace, which thereafter is refined and alloyed as required. This process requires the ready availability of molten pig iron, produced by blast furnaces.
Where molten pig iron is not available, and/or for the production of certain specialty grades of steel, an electric arc furnace (EAF) process is followed. In a typical EAF process, solid charge ingredients including raw scrap, limestone, burnt lime, iron ore and ferro alloy additives, are placed in the top-charge furnace unit. A conventional furnace unit is equipped with (1) a roof lift and swing arrangement which permits the roof to swing aside when cold scrap is charged into the furnace; (2) a rocker and rail tilting type arrangement which permits the furnace to tilt forward for tapping and backward for slagging; (3) a system for additions through the furnace roof; and (4) evacuation systems for the removal of dust generated during the steelmaking cycle.
The electrodes are supported by electrode arms and clamps, and project from overhead down through the furnace roof. The electrodes are automatically controlled by an electro-mechanical positioning mechanism. An electric arc surging between the electrodes and scrap produces heat which melts the charge and refines the steel. The molten steel is tapped, typically at about 3000.degree. F., into a ladle and cast into blooms or poured into ingot molds.
In such a process, particulate emissions are generated during (1) charging of scrap, (2) tapping of furnaces, (3) pneumatic injection of additives, (4) oxygen blowing and (5) meltdown/refining periods. This particulate, EAF dust is collected in baghouses. Even though carefully monitored landfills have been used to minimize the problems associated with EAF dust, the EPA has determined that such inorganic dust constitutes a hazardous waste. More specifically, EAF dust is currently classified as EPA Hazardous Waste No. K061 (emission control dust/sludge from the primary production of steel in electric furnaces) and, accordingly, must be managed as a hazardous waste.
As a result of this determination, the assignee hereof has actively pursued various methods for managing EAF dust. The present invention is the result of this pursuit, and comprises a chemical stabilization process which renders the hazardous constituents in the dust virtually immobile. Such process is based on the pozzolanic reaction of materials containing anhydrous alumino-silicates which, in the presence of lime, water and chemicals, adsorb and/or physically entrap the heavy metals present in EAF dust into a calcium-alumino-silicate matrix, thereby rendering them essentially immobile. The process, and the results achieved thereby, will be described in greater detail in the specifications which follow.